fix: skip external MariaDB repo on trixie, clean stale repo files

- Trixie uses native Debian mariadb packages (external repo has no
  trixie release), so wo_mysql_repo is empty and repo add is skipped
- Clean up old openlitespeed.list and mariadb.list during install so
  they get regenerated with correct GPG keyring paths

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

wo/cli/plugins/stack_pref.py

@@ -27,8 +27,8 @@ def pre_pref(self, apt_packages):
     """Pre settings to do before installation packages"""
 
     if ("mariadb-server" in apt_packages or "mariadb-client" in apt_packages):
-        # add mariadb repository excepted on raspbian and ubuntu 19.04
-        if not (WOVar.wo_distro == 'raspbian'):
+        # add mariadb repository excepted on raspbian, trixie (uses native pkgs)
+        if not (WOVar.wo_distro == 'raspbian') and WOVar.wo_mysql_repo:
             Log.info(self, "Adding repository for MySQL, please wait...")
             mysql_pref = (
                 "Package: *\nPin: origin  deb.mariadb.org"